In motor vehicles with a stop/start functionality, the internal combustion engine can be automatically stopped, that is to say switched off, when no propulsion power is required (for example when waiting at a stoplight), and automatically restarted, that is to say started up, when the driver requests propulsion power again. A stop/start operating mode of this kind reduces fuel consumption and noise emissions.
Stop/start systems have to ensure that the starter battery can maintain a voltage which is sufficient both for the current consumption in the motor vehicle during the stop phase and for restarting the internal combustion engine, after the internal combustion engine has automatically stopped. Many start/stop systems make use of a battery monitoring system which continuously measures the current flowing into and out of the battery and integrates the measurement signal, in order to obtain from this the charge state of the starter battery. If the charge state meets a predefined criterion, it is assumed that the battery can supply the required voltage. However, current measurement requires a current sensor, usually either a branch current sensor or a Hall sensor. Both types of sensor are relatively complex and add significantly to the costs of the stop/start system. In addition, it may be expedient for structural reasons to use a dedicated controller for the battery monitoring system, but this entails additional expenditure, especially since communications lines for connecting to other controllers are required.
EP 1 207 297 A2 discloses a method and an arrangement for ascertaining the ability of the starter battery of an internal combustion engine with a stop/start functionality to start, with the ability to start being estimated on the basis of the voltage drop across the starter battery which occurs during starting of the internal combustion engine. To this end, the charge state of the starter battery also has to be known, this being ascertained by a battery monitoring system.